From publisher blurb:
There is no avoiding the fact that big populations create a lot of waste. All big cities need some system to remove this waste, otherwise in short order they would be overwhelmed by filth and plagued by disease. In smaller cities this system may be as simple as a series of gutters which carries the contents of voided bladders, chamber pots, and garderobes through the open air into a local river or cesspit. More technically advanced cities often have systems of tunnels, or sewers, beneath the city. Sewers are designed to control waste water, refuse, and possibly even prevent flooding during high rains. Though they play a vital role in sanitation, sewers are often ill-maintained (out of sight, out of mind) and rarely patrolled. In fact, in most cities the only folks who are interested in the underground sewer networks tend to be smugglers and thieves, not guards or citizens.
In older cities, such sewers may not be the only structures or passages below the surface. Natural caverns, secret tunnels or vaults, local mines, garbage pits, and the remains of old cities may be intermingled with basements, cisterns, water wells, and rifts from unstable geology. In addition to the garbage and waste the city removes or stores below ground, unsavory characters use the dark pits to hide bodies, failed experiments, and contraband too risky to be caught with. The bigger the city, the more shame its residents try to sweep far, far under the rug. But nothing stays buried forever. When trash, rot, disease, magic, and forgotten ruins mix, treacherous things are bound to grow. And in time, those dangers have to be dealt with…